Search for a bound trineutron with the 3He(π−pgr;+)nnn reaction

A search for the production of a bound trineutron state has been performed using the reaction ³He(π⁻,π⁺)nnn at incident pion energies of 65, 75, and 120 MeV. No evidence for the existence of the ³n was found, and an upper limit for the production cross section of approximately 30 nb/sr (2σ confidence level) was obtained. Received: 22 October 1998     

Search for long-lived particles in 200 GeV/c proton-nucleon collisions

A search for long-lived particles (τ ? 10^?8 sec), with changes $\text{±}\text{2}\text{3}\text{, ± 1, ±}\text{4}\text{3}\text{and}\text{± 2}$, produced in 200 GeV pN collisions was performed at the CERN-SPS in a secondary beam equipped with superconducting r.f. separators. Upper limits were obtained for the production of long-lived hadrons and leptons. For charge - 1 particles the limits are at the level of 10^?7 of the pions at a mass m = 0.2 GeV and reach the 10^?11 level from $\text{3}\text{to}\text{8}\text{GeV}$. The production of light antinuclei was measured. The cross sections for ³He production are (1.3 ± 0.3) and (1.9 ± 0.3) × 10^?34 cm²/sr · GeV/c at 21 and 47.4 GeV/c respectively; for t the cross section is (7.6 ± 0.9) × 10^?34 cm²/sr · GeV/c at 23.7 GeV/c.     

Measurement of the hyperfine interaction of free75As ions In H2, He,Ne, Kr and Xe buffer environments and estimate of clolision cross sections in the eV and meV energy region

With the time-integral perturbed angular correlation (TIPAC) method the pressure dependence of the perturbation of the 121.1–279.5 keV γ-γ cascade in⁷⁵As has been investigated using gaseous H₂ ⁷⁵Se sources in different buffer environments. The obtained attenuation coefficients G₂₂ (∞) in the range of 0.55 to 1.0 were fitted with a theoretical stochastic model. In the region of low density, where the correlation time τ_C is large compared to the lifetime τ of the intermediate level of the nucleus, charge transfer collision cross sections have been evaluated between 8.1·10^?16 cm² for He, 1.3·10^?14 cm² for Xe and 1.8·10^?14 cm² for the molecular H₂ buffer gas. For increasing densities of the buffer gases the correlation time τ_C became small compared to the intermediate lifetime τ. The main effect in this region is the depolarization, and we found cross sections between 5.2·10^?15 cm² for He, 8.7·10^?14 cm² for Xe and 1.2·10^?13 cm² for the H₂ buffer gas.     

π− Production in16O +27Al at 38, 65 and 93 MeV/u

Negative pion production cross-sections are measured at 38, 65 and 93 MeV per nucleon in O + Al reaction. Rough estimations of the total cross sections are given and compared to previous neutral pion data. Double differential cross sections at 93 and 65 MeV/u show enhancement for pions with velocity close to the beam. At variance no effect is observed at 38 MeV, where π^? and π⁰ distributions are identical. Fewπ ⁺ detected at 38 MeV support this result. This indicates that at the lowest incident energy nearly the whole projectile participates in the production reaction.     

Messung der Wirkungsquerschnitte für die Linienverbreiterung von Hochfrequenzübergängen im 63 P 2-Zustand des Quecksilbers durch Stöße mit Edelgasatomen

The linewidth of high frequency transitions between Zeeman-levels of the metastable 6³ P ₂-state of mercury is measured as function of the pressure of various noble gases. The measurements are made for all noble gases in the pressure range from 10^?3 to about 2 · 10^?2 Torr. The cross sections for linebroadening due to atomic collisions are derived from the pressure dependence of the linewidth. These cross sections σ were found to be (71 ± 10) · 10^?16 cm² for He, (82 ± 10) · 10^?16 cm² for Ne, (153± 12) × 10^?16 cm² for Ar, (204±28) · 10¹⁶ cm² for Kr and (291 ± 41) · 10^?16 cm² for Xe.     

Relation of photonuclear reactions to π-meson photoproduction process

Neutron-proton pair knocking-out by photons from iron (Fe⁵⁶) nuclei is studied by measuring induced gamma activity of Mn⁵⁴ isotope. Experiments are performed in a synchrotron beam at two maximum photon energies, 150 and 650 MeV. It was found that the yield of the reaction under study at the maximum energy of 150 MeV is almost zero, whereas a significant yield of gamma active Mn⁵⁴ nuclei is measured at the energy of 650 MeV. The cross section of the neutron-proton pair production reaction on Fe⁵⁶ at an energy of 650 MeV is estimated as 4 · 10^?27 cm².     

Pion electroproduction in the 3He(e,é π+)3H reaction at intermediate energies

The differential coincident pion electroproduction cross section in the ³He(e,é π⁺)³H reaction has been measured with high resolution with the three-spectrometer set-up at the Mainz Microtron (MAMI) electron accelerator. Measurements were performed at the four incident energies E₀ = 855, 675, 600, and 555 MeV at fixed four-momentum transfer Q² = 0.045 GeV², with the pions detected in parallel kinematics. This enables a separation of the measured cross section into the transverse and longitudinal structure functions by means of the Rosenbluth method. The experimental data are compared to model calculations, in which the elementary pion production amplitude includes the standard Born-amplitudes and also delta and higher resonance terms. Three-body Fadeev wave functions are used and the final state interaction of the outgoing pion is taken into account. The calculation describes the data only after medium modifications of the delta and of the pion are included.     

Muo-production of neutrino pairs and the number of generations

The cross section for production of neutrino pairs by high energy muons in the nuclear Coulomb field is calculated analytically. For right (left)-handedμ ^?(μ ⁺) helicity, the process is only mediated by neutral currents, which opens the possibility to look for the number of generations. Assuming three generations, the calculated cross section turns out to be 1.1×10^?40 cm² for⁵⁶Fe and 6.6×10^?40cm² for²⁰⁸Pb at an incident muon energy of 300 GeV. Some comments about the equivalent photon approximation are made.     

Photoemission of protons from4He in theΔ-resonance region

High resolution proton energy spectra from the⁴He(γ,p) reaction have been measured with tagged photons in the range E_γ = 130?525 MeV using the large Mainz NaI(T1) spectrometer atΘ _p ^lab =37.1°. Three separate reaction channels were identified, viz.⁴He(γ,p)t two-body breakup,⁴He photodisintegration via two-nucleon photon absorption processes and the quasifree pion production channel. Differential cross sections are presented for each of these channels as a function of photon energy. The sum of the two-nucleon photon absorption and the quasifree pion production differential cross sections, in the CM system, resembles that of the corresponding free-nucleon differential cross section when Fermi motion is taken into account.     

Electron impact desorption of Xe from the tungsten (110) plane

The electron stimulated desorption of Xe adsorbed on the clean and on oxygen and CO covered tungsten (110) surfaces has been investigated. Only neutral Xe desorption was observed; for Xe on clean W a very small initial regime with cross section 10^?17cm² is followed by a slow decay with cross section 3×10^?19cm². The Xe yield varies nonlinearly with coverage, suggesting desorption from edges of islands or from sites with less than their full complement of nearest neighbor Xe atoms. Desorption from oxygen or CO covered surfaces results in an apparent desorption cross section identical to that of the underlying adsorbate. This results from a kicking off of Xe by electron desorbed O or CO. The true cross sections for these processes are ～10^?14cm² for Xe-0 and ～10^?15 cm² for Xe-CO. Some speculations about the mechanism, particularly the absence of ions are presented.     

Inclusive π4He scattering and the πN interaction in the nuclear medium

Double differential cross sections for the inclusive reaction ⁴He(π, π′)X have been measured for six pion energies between 90 and 320 MeV and an angular range from 30° to 135°. The data are interpreted in terms of the Δ-hole formalism. Medium corrections to the free π-nucleon scattering operator are important for a quantitative understanding of our results. Total inelastic cross sections are obtained and estimates for the absorption cross section are given.     

Scattering of negative pions on helium

Differential cross sections for negative pion scattering on ⁴He have been measured at five pion kinetic energies between 110 and 260 MeV in the angular range from 5° to 180°. Total cross sections have also been measured at eleven energies between 67 and 285 MeV. The differential cross sections have been fitted with a phenomenological expression for the nuclear scattering amplitude. Conventional phase shifts have been reconstructed starting from the parameters of the fits.     

Coherent production ofρ − mesons in charged current antineutrino-neon interactions in BEBC

Coherent production ofρ ^? mesons in charged current antineutrino interactions on neon nuclei is studied in the BEBC bubble chamber exposed to the CERN SPS wide band beam. The cross section is measured to be (95±25)·10^?40 cm² per neon nucleus, averaged over the beam energy spectrum. The distributions of kinematical variables and the absolute value of the cross section are in agreement with theoretical predictions based on the CVC hypothesis and the vector meson dominance model.     

Photoemission of protons from 4He in the ⊿-resonance region

High resolution proton energy spectra from the ⁴He(γ,p) reaction have been measured with tagged photons in the range E_γ = 130–525 MeV using the large Mainz NaI(Tl) spectrometer at \([Theta _p^{lab} = 37.1^circ ]\) . Three separate reaction channels were identified, viz. ⁴He(γ,p)t two-body breakup, ⁴He photodisintegration via two-nucleon photon absorption processes and the quasifree pion production channel. Differential cross sections are presented for each of these channels as a function of photon energy. The sum of the two-nucleon photon absorption and the quasifree pion production differential cross sections, in the CM system, resembles that of the corresponding free-nucleon differential cross section when Fermi motion is taken into account.     

The reaction 14C(3He,n)16O and the coexistence model of 16O

The reaction ¹⁴C(³He, n)¹⁶O has been measured at a ³He bombarding energy of 25.4 MeV. The zero-degree differential cross section for the excitation of the three low lying 0⁺T = 0 states, at energies 0.0, 6.05 and 12.05 MeV are, respectively, 1.33 ± 0.10, 0.49 ± 0.10, and 0.50 ± 0.10 mb/sr These measured cross sections are in rough agreement with single-step zero-range DWBA calculations using an empirically determined ¹⁴C ground state wave function and in which the Brown and Green coexistence-model wave functions are used to describe the ¹⁶O 0⁺ states. The angular distribution of the transition to the ground state is measured between 0° and 32°.     

Direct Measurement of 12C+4He Fusion Cross Section at E cm = 1.5 MeV at KUTL

¹²C+⁴He fusion reaction at E _cm  = 0.3 MeV is one of the main reaction in ⁴He-burning in stars, and is very important in nuclear synthesis. However the fusion cross section has not been determined yet in spite of 40 years efforts in the world. Recently we succeeded in the measurement at 1.5 MeV. We have a plan to make direct measurement of the total fusion cross section down to 0.7 MeV at Kyushu University Tandem accelerator Laboratory (KUTL), and to determine the cross section at 0.3 MeV by extrapolation.     

Measurement of the cross section of the reaction pp → pp π0 in the region of dibaryon resonances

The energy dependence of the cross section of the neutral pion production is measured in the range 600÷900 MeV. No unambiguous evidences for the existence of broad isovector dibaryon resonances were obtained. The isoscalar cross section of the pion production does not reveal any structure, predicted by the theory with resonances.     

Measurement of the cross section of the reaction pn → ppπ− in the energy region of dibaryon resonances (500–1000 MeV)

The cross section of the reaction pn → ppπ^? has been measured in a deuterium bubble chamber at 9 incident energies in the range 500–1000 MeV. The energy dependence of the isoscalar cross section of single pion production is obtained. It is shown, that if an isoscalar dibaryon resonance exists with the mass in the range 2.17–2.30 GeV its elasticity should be smaller than the theoretical predictions available.     

Elektronennachlieferung in Niederdruckentladungen durch metastabile Stickstoffmoleküle und deren Stoßlöschung durch Fremdgase

Oscilloscopic studies of pre-breakdown discharge currents in pure nitrogen and mixtures of nitrogen with other gases show that metastable N₂ molecules influence the discharge by releasing secondary electrons when they collide with the electrodes. By this slow process an after current is produced, which lasts a time of milliseconds. It was found that the diffusion coefficient of the metastable N₂ molecule is 167 cm² s^?1 ±15% at 1 Torr and 20°C, its radiative life time is larger than 40 ms, and its cross section for deexciting collisions with ground state N₂ molecules is smaller than 7 · 10^?22 cm². There is strong evidence that the metastables are in theA ³Σ _u ⁺ state. From measurements in gas mixtures cross sections for deexciting collisions with other molecules are derived. The results are for O₂ 1.9 · 10^?17 cm², for CO 1.5 · 10^?17 cm², for H₂ 2.5 · 10^?20 cm², and for CO₂ smaller than 5 · 10^?20 cm².     

Anregung des 7,66 MeV-Niveaus in Kohlenstoff-12 durch unelastische Elektronenstreuung

Using the facilities of the Darmstadt electron linear accelerator the cross section for the excitation of the 7.66 MeV level in Carbon-12 was measured for momentum transfers between 0.25 and 0.55 fm^?1. Extrapolation of the measured cross sections for vanishing momentum transfer yields a partial width for the decay to the ground state through internal pair production of (7.3±1.3)·10^?5 eV. The dependence of the reduced transition probability on momentum transfer is discussed and compared with theoretical predictions.